We describe here a continuous perfusion bioreactor system that enables a population of unselected human mononuclear bone marrow cells obtained from adult donors to expand up to 20 to 25-fold over a two-week period. Colony-forming units of granulocyte-macrophage (CFU-GM) progenitor cells expand 10 to 30-fold. These expansions depend on the gas phase oxygen concentration, the seeding density and time of cell harvest. Under operating conditions that allow for good cell proliferation, 3 to 4 million mononuclear cells can be obtained per square centimeter, with 0.5 to 0.8% being progenitor cells. Autologous human sera supported cell expansion as efficiently as animal sera. Increasing the size of the perfusion system to produce a clinically meaningful number of CFU-GMs could have important applications in bone marrow transplantation therapies.